Not applicable.
This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to a multilayer thermal barrier coating (TBC) system characterized by a low coefficient of thermal conductivity.
The use of thermal barrier coatings (TBC) on components such as combustors, high pressure turbine (HPT) blades and vanes is increasing in commercial as well as military gas turbine engines. The thermal insulation of a TBC enables such components to survive higher operating temperatures, increases component durability, and improves engine reliability. TBC is typically a ceramic material deposited on an environmentally-protective bond coat to form what is termed a TBC system. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.
Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by air plasma spraying (APS), flame spraying and physical vapor deposition (PVD) techniques. TBC""s formed by these methods have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries of the TBC microstructure. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity throughout the life of the component, including high temperature excursions. However, the thermal conductivities of TBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs. A TBC with lower thermal conductivity would allow for higher component surface temperatures or reduced coating thickness for the same surface temperature. Reduced TBC thickness, especially in applications like combustors which require relatively thick TBC""s, would result in a significant cost reduction as well as weight benefit.
The present invention provides a thermal barrier coating (TBC) and method by which a low thermal conductivity of the TBC is maintained or even decreased as a result of a post-deposition high temperature exposure. The TBC is part of a TBC system that includes a bond coat by which the TBC is adhered to a component surface. The TBC of this invention preferably comprises an inner layer on the bond coat and an insulating layer overlying the inner layer. According to one aspect of the invention, the inner layer preferably contains yttria-stabilized zirconia (YSZ), while the insulating layer contains barium strontium aluminosilicate (BSAS; (Ba1!xSrx)Oxe2x80x94Al2O3xe2x80x94SiO2) The thermal conductivity (Tc) of BSAS is approximately equal to that of YSZ. However, the thermal conductivity of BSAS has been surprisingly observed to decrease with sufficiently high temperature exposures, with the result that, though having similar as-deposited thermal conductivities, BSAS can become a better thermal insulator than YSZ if it undergoes an appropriate thermal treatment.
Because BSAS has a low coefficient of thermal expansion (CTE) (about half that of YSZ), and therefore a BSAS coating may not adequately adhere directly to a metal substrate. In addition, alumina (Al2O3) scale that forms on aluminum-containing bond coats may react with the silica content of the BSAS coating to form silicate-type phases that would further diminish the adhesion of the coating. Therefore, the present invention provides the YSZ-containing inner layer, which has a sufficiently high CTE to mitigate the CTE mismatch between the BSAS-containing insulating layer and the underlying metal substrate (e.g., bond coat).
In view of the above, the present invention provides a TBC with a low-Tc outer coating (BSAS) whose thermal conductivity is reduced from its as-deposited Tc through an intentional high temperature thermal treatment. While not wishing to be held to any particular theory, the thermal conductivity of BSAS is believed to decrease with temperature exposure as a result of grain shape changes driven by the surface energy reduction, which causes pores to form in the BSAS coating. The resulting porosity decreases the thermal conductivity of the BSAS coating, with the result that the BSAS coating has significantly lower thermal conductivity than a conventional YSZ coating of the same thickness. As a result, a TBC containing a BSAS insulating layer in accordance with this invention is particularly suitable for thermally insulating components intended for demanding applications, including advanced gas turbine engines in which higher component surface temperatures are required. Alternatively, the lower thermal conductivity of the TBC allows for reduced coating thicknesses for the same surface temperature, resulting in a significant cost reduction as well as weight benefit.
Other objects and advantages of this invention will be better appreciated from the following detailed description.